No notifications push or otherwise on Xiaomi Redmi Note 13 Pro Plus 5G

Tried all settings in app and all settings on phone Hyperos. Have used K-9 Mail for many years on other phones, never had this issue, always been rock solid performance from this email client.
K-9 Mail receives emails when I open the app, otherwise nothing from notifications until the app is opened.
Using another email client for now works fine on this phone, but I want K-9 Mail back to working.

Many AOSP derivates from China have super strict power saving setting. Just Google “[Chinese Manufacturer’s Name] Power Saving App does not work”…

I had the same problem a while ago with the same brand of smartphone, installing the latest beta version of K-9 solved it.
Meanwhile I ditched that smartphone brand because also other apps did not work correctly. With a new phone from another brand everything is working fine again …